Questionable Life

Finding evidence of ancient life on Mars may be even more difficult than we thought.

If astronauts manage to touch down on Mars sometime in the next century, one of their priorities will be to determine whether life ever existed there. And one of the best pieces of evidence, most researchers have thought, might come in the form of rocky structures called stromatolites. On Earth, stromatolites are thought to have been formed by the activities of the earliest living things--presumably simple bacteria. Martian stromatolites would then be a sign that Mars too had once supported life. Now, however, two researchers at mit argue that at least some stromatolites can be formed by purely physical processes and thus cannot always be relied on as definitive evidence for the past presence of life.

Some stromatolites clearly are the result of biological growth. Indeed, they can be observed growing even now in the Bahamas, Australia, and elsewhere. Generations of sticky mats of bottom-dwelling bacteria growing in warm shallow waters trap layer upon layer of sediments that eventually harden into rock, forming stromatolites. In cross section the sediment layers have a characteristic wavy pattern. Most geologists had assumed that ancient stromatolites, even those that contained no fossil bacteria, formed in the same way. Some 3.5-billion-year-old stromatolites in Australia, for example, have been taken as evidence--although not the only evidence--of early life on Earth.

The work of MIT geologist John Grotzinger and geophysicist Daniel Rothman raises questions about such assumptions. Grotzinger and Rothman are not the first to suggest that stromatolites might form in the absence of life, but they are the first to analyze carefully how such a process might occur.

They began by making photographs of cross sections of stromatolites from a fossil reef in northwestern Canada and converting the photos into digital form for computer analysis. The geologists then tried to reproduce the same digital data using a computer model for stromatolite growth that didn’t involve any biological processes. Instead, Grotzinger says, their model was meant to duplicate how chemicals and sediments might settle out of water in a hot ancient ocean rich with the calcium carbonate typically found in stromatolite layers. The model shows that the wavy layers of calcium carbonate could have formed as the mineral settled into shallow depressions on the seafloor. The calcium carbonate crystals would have piled up in uneven bands along the sea bottom.

The results of the model were indistinguishable from the digital data the two researchers had collected from the Canadian stromatolites, which means that investigators cannot substitute just any stromatolite for fossil evidence in the search for ancient life, whether on Earth or Mars. Stromatolites alone can never be used as definitive evidence for the presence of life, says Grotzinger, simply because their geometric aspects are going to be identical to things that can be produced abiologically.